Silica sols and preparation thereof from metallic silicon



Patented Oct. 21, 1952 SOLS AND PREPARATION THEREOF FROM METALLIC simooN Joseph H. Balthis, Mendenhall, Pa., assignor. to

E. I. du Pont de Nemoursand Company, Wilmington, DeL, a corporation of Delaware SILICA NoDrawing. Application June 13,1951,

SerialNo. 231,443

Thisinvention relates. to silica sols and to a method of preparing same and, more particularly, to the preparation of aqueous colloidal solutions of: silica wherein the silica particles differ from those heretofore known. This application is a continuation-in-part of applicant's copending application Serial No. 95,369, filed May 25, 1949, and entitled Preparation of Silica Sols, now abancloned.

Aqueous silica sols are of considerable technical. importance, for example in the treatment of hosiery to impart snag resistance, as fillers for white rubber, non-skid adjuvants to waxes, textile sizing and finishing agents, ingredients in emulsifying agents and in many other applications. These sols have generally been prepared from sodium silicate by treatment with acids or ionexchange resins. No method to prepare silica sols. directly from silicon has heretofore been known. Furthermore, the silica sols heretofore prepared have'been characterized by having silica particles which are relatively dense and which exhibit relatively low reactivity in. alkaline medium toward depolymerization, characteristics which are not advantageous in certain fields of use.

This invention has as an object a new method for preparingsilica sols. A further object is a method by means of which silica sols are easily and economically obtained from metallic silicon. A still further object is a method for preparing silica sols which is convenient in practice and which yields silica sols of high concentration and of controllable particle size. In addition, the invention has as an object the provision of a new aqueous colloidal solution of silica and of new silica particles which are highly porous and exhibit outstanding activity toward depolymerization in alkaline medium, thus permitting formation of salts by reaction atroom temperature with strong organic bases. Other objects will appear hereinafter.

The above objects are accomplishedby reacting finely-divided silicon metal with water which contains at least 1% by weight of ammonia and which is present inamount providing at least 0.1 gram-mol of ammonia per gram atom of silicon. Hydrogen is evolved. during the'reaction and the metallic silicon isconverted to a stable colloidal solution-of silicon dioxide.

Thepurity or reactivity of the silicon has an influence on the reaction. Commercial silicon of ordinarygrladereacts, ifit is finely divided shortly beforesuse, but ill-(i085 so. only at a slow rate; Silicomofivery higlrpurity ismore satisfactory from 11. Claims. (Cl. 252313) this standpoint since it usually reacts at a much faster rate, but silicon of high purity is very expensive to use- However, silicon ofany grade reacts: at a satisfactory rate if it is first subjected to certain treatments which renders the metal more reactive,- presumably by removal of a film of. silicon dioxide from the particles. The: pre activation treatment may beca-rried out, for ex.-

ample, by washing the metal (preferably ina,

finely-divided form) with aqueous hydrofluoric acid, the latter being suitably at concentrations between 1% and 458%, or with aqueous solutions of ammonium bifiuoride; or by carrying outthe reaction with ammoniain a vented-ball mill wherein. the metal is subjected. to continuous grinding; or, less effectively, by grinding; the metal, mechanically or otherwise, just prior to use in the reaction; or by a combination of these methods. It is preferred that the silicon metal be reduced to a small particle size, e. g., between and 320 mesh or finer. Larger particles will re act, but more slowly. Freshly ground silicon is more reactive when prepared from relatively large particles, e. g., particles the size of a pea or a marble, than from coarse powders, probably because there is less initial superficial silica. In addition to elementary silicon, itis possible to use certain silicon alloys such as ferrosilicon containing or more silicon.

The maximum concentration of ammonia in the: aqueous ammonia reacted. with the finely divided silicon isnotmaterial except from the standpoint of particlesize of the silica in theresultingsol. Water containing 28% of ammonia (the commercial ammonium: hydroxide) ormuch more if desired, e. -g-., 35%, may be used but there is no technical advantage in using high concentrations In' fact, it is often preferable to'use as low concentrations of ammonia as is consistent with good reaction rate, for the reason that the particle size of the silica in the sols decreases with decreasing concentration of ammonia, and sols of low particle size are often preferable for many technical applications. The. preferred range af ammonia concentration in the aqueous system lies between 1% and 10%. Likewise, the ratio gram-mole of. ammonia/ gram atoms of silicon may vary between. about 0.1 and 30 or even more, but it is preferably between 0.5 and6. Some reaction (conversion of silicon to silica) can be observed at somewhat lower ammonia concentrations or ratios than those specified, but the reaction is then so slow and incomplete that it is unpracticable.

The reaction temperatureis not critical; asw-the scope.

reaction proceeds even at temperatures as low as C. However, the rate of reaction increases with increasing temperature, and it is preferred to operate at temperatures between 20 C. and90 C. Higher temperatures, e. g., 100 C.-'150 C., can be used but there is in general no advantage in doing so because of unnecessary loss of ammonia. It is, of course, possible to operate at elevated temperatures, e. g., up to 200 C. or higher, with the use of pressure equipment.

Agitation during the reaction is not essential but it is desirable, as the rate and extent of reaction are increased thereby. Agitation can be combined with grinding, as when the reaction is carried out in a ball mill.

The progress of the reaction can be followed by the gradual disappearance of the metallic silicon and the rate of evolution of hydrogen. In general, an induction period is observed, after which the reaction starts and increases in rate, then becomes slower, possibly because a film of silica is deposited on the unreacted silicon. Thus, it is often advantageous to interrupt the process when one-third to two-thirds of the silicon has reacted and to recover the unreacted metal by filtration or centrifugation. It can then, if desired, be reactivated, for example by washing with aqueous hydrofluoric acid. Depending on conditions such as temperature and state of activation of the metal, substantial conversion to silica usually occurs within 4 to 48 hours from the start of the reaction, or much more rapid reaction can be achieved by operating in a ball mill. It is in general desirable to carry out the reaction until at least of the silicon has been converted to silica. Continuous or semi-continuous operation is possible by withdrawing the sol continuously and adding ammonia as required.

The silica sols produced by the process of this invention show very good uniformity of particle size when examined under the electron micro- The particle size obtained with 1-5% ammonia solution is in the very low range of 8 to millimicrons (80-150 angstrom units), as measured by light scattering technique. Such a fine particle size is not ordinarily obtainable by the methods heretofore known for preparing silica sols. In general, the particle sizewill not exceed about millimicrons when the silica sols are made according to this invention. Moreover, the sols of this invention are highly stable. They can be concentrated without difficulty merely by boiling ofi part of the water and the excess ammonia. Thus, it is possible to obtain sols having solids contents as high as -50%. taining as much as 25-35% solids and even more are extremely stable and show no tendency to gel upon prolonged storage. In addition, the silica particles are highly porous and exhibit outstanding activity toward depolymerization in alkaline medium.

The invention is illustrated in greater detail in the following examples wherein all proportions are by Weight unless otherwise stated:

Example I The silicon used in the experiment was a substantially chemically pure material containing only traces of magnesium and copper by spectrographic examination. It had a conductivity of 0.01-1 reciprocal ohm/centimeter, as compared with a value of 15-30 for the best commercial products.

This material was ground to a fine powder and 1.058 parts of the freshly ground metal was placed Sols conin a vessel with 45 parts of commercial aqueous ammonia containing 26-28% NHs by weight, the

reaction mixture being maintained without agitation at 20-25 C. Strong eifer'vescenc'e commenced almost immediately. The liberated gas was hydrogen containing some entrained ammonia. After 21 hours the reaction mixture was filtered. From the weight of the unreacted metal it was calculated that 27.7% of the silicon had been converted to silica, which was present in the filtrate as a colloidal sol at a concentration of 0.018 g./cc., as determined by evaporation of weighed samples.

Example I I The same silicon as that described above was used. Two parts of freshly pulverized metal was reacted with parts of aqueous ammonia containing 26-28% NHs at 20-25 C. for hours, the reaction mixture being mechanically agitated for about two-thirds of this time. From the weight of the unreacted metal it was calculated that the reaction had proceeded to the extent of 69.3%. Titration of samples of the reaction mixture at intervals showed that the ammonia concentration decreased from 13.95 N to 10.43 N during 150 hours. A control containing no silicon showed practically the same loss of ammonia, from 13.95 N to 10.94 N, which indicates that the loss of ammonia in the conversion of silicon to silica sol is accounted for by normal evaporation and entrainment with the hydrogen evolved.

Example III The pure silicon described above was used. A mixture of 6 parts of freshly pulverized metal and parts of aqueous ammonia (26-28% NHs by weight) was ieacted for 24 hours at 120 C. in a sealed vessel at the autogenous pressure of the aqueous solution. From the weight of the unreacted silicon it was calculated that the reaction had proceeded to the extent of 47.4%. There was obtained an aqueous silica sol containing 0.052 g. of silica per cc. Electron photomicrographs showed that the particles were remarkably uniform in size.

When the above sol was concentrated by boiling to one-fifth of its volume, the remaining concentrated sol was completely stable on several weeks storage. The sol had a pH of about 5 when concentrated to half its original volume.

Example IV A. sample of ordinary commercial silicon was round to a fine powder, washed thoroughly with 48% aqueous hydrofluoric acid and rinsed with water. When this washed silicon was treated with 28% aqueous ammonia at 20-25 C., reaction started immediately with evolution of hydrogen and formation of a silica sol. After 7 hours the amount of hydrogen evolved indicated that the conversion was about 36% complete. When another sample of the same silicon, ground but not washed with hydrofluoric acid, was treated with 28% ammonia, the reaction was slower. It had proceeded only to the extent of about 6% after 7 hours.

The effects of the concentration of ammonia on the reaction and product are shown below in tabular form. All of the tabulated runs were carried out with silicon metal of the quality described in Example I, which, to insure maximum activity, was activated by stirring with 48% hydrofluoric acid, filtering, washing in turn with water, alcohol and ether and finally drying in an Ammonia Content Diameter of Particles (Millimicrons) Wt; Average Molecular Wt:

Grammols per Gram atom of Bun Percent y Weight It will be seen from the above table that. the ammonia concentration, down to a value of about 1%, has relatively little effect on the reaction rate. It has, however, an important effecton the particle size-of the silica in the sols.

Example. .V

Siliconiwhi'ch had been ground and passed through a 320 meshscreen was agitated with 20% aqueousammonia bifiuoridetfiltered, and .washed in" turni with water, alcohol and ether and. dried in a stream of nitrogen. Two partsof this silicon were added to 90 parts of 28% aqueous ammonia, and the mixture was allowed to stand for 12 days. Unreacted silicon was separated by filtration. The resultin sol had a solids content of 0.0064 g./cc., as compared with a solids content of 0.0022 g./cc. when the silicon was not previously treated with ammonia bifiuoride.

Example VI A sample of silicon of the same high degree of purity as described in Example I was finely ground and allowed to age for 4 months. Upon treatment with 28% aqueous ammonia at 20 C.-- 25 C. for 2-3 days, reaction took place to some extent but the final silica sol contained only 0.215 g. of solids per 100 cc. When another sample of the same silicon was finely ground just prior to use-and treated under the same conditions, the silica sol contained 0.626 g. of solids per 100 cc. When a third sample of the same silicon was ground, washed with 48% hydrofluoric acid as described in Example IV and then treated under the same conditions, the silica sol contained 2.05 g. of solids per 100 cc.

The'process described herein has the important advantage of permitting control of the particle size of the resulting silica sols, since the particle size is in a large measure proportional to the amount and concentration of ammonia employed. Moreover, in contrast with former methods of converting silicon to silica sols which involve the use of sodium silicate as an intermediate, the

'present process employs avolatile agent (ammonia) which apparently serves as'a catalyst for the reaction of water with silicon and can readily be expelledf-rom thereaction product. Removal 6 of excess" ammonia can: be acc'omplishedisimpl'y by boiling the" silica so'l, if desire'd under. reduced pressure; or by passing'a'current of airiori inert gas through the sol, thus' l'eaving' an aqueous system containing colloidal" silica particles 'sub'zstantially free of extraneous ions. Suchsystems are" highly advantageous for the-preparation of silica-gel for use in catalytic processes since .they

are free of sodium-and other catalytically objectionable ions: If the presence of smallamounts of extraneous material is not detrimental, the ammonia need not be-r'emoved' 'or only partly so: or it can be neutralized'by' additionof fixed acids, or'of-carbon dioxide or sulfur'dioxide. It is someitimes desirable to add acids or non-volatile alkali to assist in controlling the molecular weight and particle size of the silica during, concentration and aging:

As already indicated; the silica solsprepared by the process of this invention are strikingly different from those heretofore known, chiefly in that the silica particles are much more porous and possess such high activity toward depolymerization even in very dilute alkali at room temperature. This high activity toward depolymerization permits formation of salts by reaction at room temperature with strong organic base-sand is especiallyvaluable'for. reaction with thermally unstable organic bases; An advantage, of the porous. character of. the silica particles. is that they give drypowders. ofilow density particularly I adapted-for useas fillersin rubber and plastics;-

For the purpose or: accurately characterizing the unique features of the aqueous colloidal solution of silica oi the. present'invention, the following definitions of terms andtests are given:

Colloidal solutions.A colloidalsolutionis one in whichthe solute particles. areoflcolloidaldimensions, i. e., particles. having an .averagediameternot exceeding aboutJOOmillimicrons. nor lessthanabout 1 millimicron.

Average: diameter.The. number. average diameter, (Zn, of particles: can. be directly observed by meansof an electronmicroscope. The diam.- eter of a, large number. of particles ismeasured, measurements being made onlyonthose particles which are lying insuch away asv to present more than a. semicircle. of cross-section. By counting and measuring a'largenumber (several hundred) of particles which are lying in. such a position as to present at least a Semicircleof silhouette, the average particle diameter. canbe determined. 'In counting. such particles, the

for example, 10to 15-millimicrons,,l5 to20millimicrons, etc. The number average particlediameter, dn, isthencalculated from the number of particles found in. each of these groups, and may be expressed mathematically. as. follows:

' where m is the number of particles in the z'th range of sizes, the mean diameter of'which is di,

and-7c isthe-numb'er of size ranges. I

External specific surface xareaL-This surface'is the-outer" surface bounding the silica particles as calculated from electron micrographs; The same P3111018" count is used as wasdescribedun- 'der "average particle diameter. The surface area of the particles within each group measured is calculated'from the average diameter of the group and the number of particles within the group. The total surface area of all of the "counted particles may then be calculated by sumas that of the total collection of counted particles. This may be expressed mathematically as follows:

where the symbols have the same meaning as described above, The specific surface area, Sc,

expressed as square meters per gram may thenv be calculated from as, measured in millimicrons, taking the density of silica as 2.2 grams per 00., which is the density of amorphous, anhydrous, non-porous silica, as follows:

' Specific surface area by nitrogen adsorption.-

This is the area of the external surface plus I that of the internal pore surface per gram of silica, asdetermined by nitrogen adsorption. The method used for measuring specific surface areas by low temperature nitrogen adsorption is described in A new method for measuring sur- Y cific surface areas.

Specific surface area by adsorption of methyl red dye.This test indicates the surface area per gram of silica which is available to the methyl red molecule. For this test, it is necessary that the sample be dried in a special manner-at least '4 hours at 110 C., in an air oven, but in no case at a higher temperature than 150 C. This special limitation on drying is necessary, since it is believed that methyl red is adsorbed only on a hydroxylated surface, that is, a surface covered with silanol groups. The methyl red adsorption test is carried out by agitating in 25 milliliters of an anhydrous benzene solution containing 0.6 to 0.7 gram of the acid form of methyl red, paradimethylamino azobenzene orthocarboxylic acid, (CH3) 2NC6H4;NC6H4COOH, per liter, a suspension of a few tenths of a gram of the dried silica sample to be tested. gram of this sample should be used in the test. Appreciably less must be used with voluminous samples, to avoid getting a mixture too thick to handle. Within the latter limitations, however,

No more than about 0.7

the amount of sample used should provide as near as possible a total available hydroxylated surface area of 10 square meters in the test. The test mixture is agitated for about two hours at about 25 C., to reach equilibrium conditions; an equilibrium concentration of 400 milligrams of dye per liter insures saturation adsorption.

The decrease in dye concentration inthe benzene solution is determined by adsorption spectrophotometric observations at 4750 A. of both'the original and equilibrium benzene solutions of methyl red. The specific hydroxylated surface area in square meters per gram is calculated from the formula:

grams dye adsorbed X 1 16 X 10 X Avogadros No.

grams silica employed Xmolecular Weight of methyl red where the covering power of each. adsorbed methyl red molecule is approximately 1.16 square millimicrons.

This method apparently measures the external specific surface area, since the values obtained agree within 125% with the external surfaces as measured by electron micrographs.

Porosity.-Porosity is indicated by the fact that the specific surface area by nitrogen adsorption is larger than that calculated from particle diameter measured in electron micrographs. The difference between these areas may be considered to be the area of the pores. These pores are apparently so small that they are not only not visible in electron micrographs, but are not appreciably penetrated by the methyl red dye.

Specific depolymerizatz'on rate.-This is a measure of the activity of the silica surface toward hydroxyl ions, and is measured by the rate of solution or depolymerization of the silica to monomeric form, divided by the specific surface area of the sample tested.

The depolymerization, or solution of polymeric silica in the presence of hydroxyl ions, results in the production of monomeric sodium silicate. The reaction of this monosilicate with acid molybdate reagent prepared from ammonium molybdate and sulfuric acid results in theformation of a yellow silicomolybdate complex,

It has been demonstrated that the color intensity of this complex is proportional to the amount of monosilicic acid which has reacted with the molybdate reagent.

Harmon, J. Phys. Chem., 31, 616-25 (1937), has shown that this complex molybdic acid reacts with ionic or crystalloidal silica, but not with colloidal silica; thus, the use of excess alkali in the standard analytical procedure for the determination of silica by the molybdate color reaction is recommended in order to convert all of the silica to the soluble or crystalloidal form.

Not only does the molybdate reagent react rapidly with monomeric silicic acid, but the acid nature of the reagent practically arrests depolymerization of any high molecular weight colloidal silica present when an aliquot of the silica sample is mixed with the acid molybdate reagent. The reaction of this agent with monosilicic acid can therefore be used to determine the amount of monomer present in the depolymerizing solution at any given time.

The molybdate reagent is prepared as follows: A hundred grams of (NH4)5MO70244H2O is dissolved in distilled water and is then diluted to 1 liter. Just prior to use, 40 milliliters of this reagent are added to milliliters of 1 normal 9 sulfuric acid and 860 milliliters of distilled water. This strength molybdate reagent provides a very convenient reading on the 'Beckman Model DU photoelectric quartz spectrophotometer (which is used to follow the color development), when mixed with enoughsilica sample to result in a maximum "silica concentration of 1 milligram per 50 milliliters total in the color developing a solution. For solutions o'fthis maximum silica concentration, anroptical .density value ,of 01720 is obtained.

The .depolymerizationrofja silica sol is carried outat 30 Cxin 0.01 :;normal sodium hydroxide solution. It :is necessary, for ;this ,test, -;to case deionized .silica ;-.sols. Therefore, :the sols vare first :deionized by .passage through la sulfonic type :cation .exchange resin, usually ".Nalcite 'HCR 1(a :nuclean sulfonated polymer .:.of .-=styrene containing divinyl benzene-and having .an,-. exchange :capacity t of 5.2.8. ki1ograins of CaCOa per cu. ft. of :resin. =.Ion Exchange Theory and-1Application, :F. :C. :Nachod, Academic iPress, :Inc, New York, 1949, .p. 385, ct seq), (followed by passage through l a :moderately, but not .qstrongly basic anion exchange resin, such as Amberlite 1124B (a modified phenol formaldehyde polyamine condensate containing 14% nitrogen and 39.9% water in the basic "form of the resin and .having an exchange capacity of 10 milliequivalents :per gram of resin. Keenin andMeyers, J. A. C. S., vol. 69, p. 2874 (1947)). After deionization, the silica sol is.,.diluted to 0.2% silica With distilled water. Then 10 milliliters of this "silica sol are'pipetted" into -90 milliliters'ofsodium hydroxide solution prepared from 80 'milliliters of water'and 1-0 milliliters-of 0.1-nor-mal='so'dium 'hydroxide, soas to -result in 100 millilitersof 0301 normal sodium hydroxide containing 0.02% silica at"30- C.

For measurement of silica powders, the procedure consists of-acid'wa'shing-with hot con- -cent-rated hydrochloric acid, followed bysmall successive portions -of distilled-water, followed "by acetone washing and drying at 110 C. Twenty milligramsof-the-=dried powder-is accurately weighed and .transferred' 'to loo-mil- "liliters of Dbl-normal sodiumhydroxidesol-ution maintained at '30" C. 'Theslurry is-ag-itated vigomu'sly by 1 bubbling with i nitrogen -gas saturated with Water vapor at 30 C.

The procedureionthe "measurement of depolymerization rate is carried out as follows:

At appropriate intervals, .determinedrby ;.the -ra-te of 'depolymerization reaction, usuallybe- 1 tween -5 andv .90 minutes, five. milliliter. aliquots are withdrawn from the. depolymerizing .solution, :andi-pipetted into :a clean? .flask. Then 455 milliliters of thermolybdate reagent-which has been freshly prepared is added.tot-herflasband the "mixture "thoroughly shaken. "The "time-of mixing with the lmolybdate reagent is taken as the time at Which depolymerization is arrested. .Theamountof monomer. present. at. any time can .thenlbe calculated from the optical .density lrea'dingras it-is. obtained on the .Beckman ;spec- -trophotometer, provided the :finalcolorv forzeom- Yplete, -Idepolymerization (100% monomer) ,is ..l:nown.

The optical .density of the solution measured at various time intervals at a'wavelengthof 400 millimicrons with a slit width of 0.03 millimeter, is' used, with. the final color value ,to determine the'percentage monomer present. .In general, the extinction or optical density is measured two "minutes after "the initial "mixing" of" silicic" acid 10 th .m lybdate ea ent, a d .is. .fo 1. .we .-2 minutesat half minute intervals ,until a constant l r valu s ebtained- Th re e ence. so utien for these measurements isthe color developing reagent itself. If the silica. sol is turbid, however, ablank composed of .5 milliliters-of the sol and .45 ,milliliters .of distilled water should also .b.e run, and the ,values so obtained subtracted from the optical. density .for the sample, to .ob-

tain a corrected optical densit ,for the ,.1 otal .s ructed. .T e p rc nte l c pres as. .m9j

omer. is..usually,plotted. as. the ordinate, while the timein minutesis-p tte a ith abscissa .jlhe s ope of thel nesoohta ne a thenjb u e to ca cula e th a e o .monomerior netien ,re' minutes of time.

The specificdepolymerization rate is defined as the rateof monomer formation per,100 minutes of time, divided by the specific surface area of th dep ly riz ne parti l a th tim .Q measurement of rate of monomerformation. Stated mathematically Are-11min where Kis the specific udepolymerization-rate,

(dm/dtlmo is therate'of monomer formation per 100 minutes-of time, 'Anistthei original :specific surface-area-of the silica/tested, .-and .is i fihe -fraction. ofztotal silica converted'to monomert-at the 1 time. t. =.This rate is not ,necessarily a-constant for-.as iven silica .samplemm ma very th ou hout he c i-11 1.yme zationre he sample- .The products .ofrthis invention have= .high

specific depolymerization rate in-cOmQarison with :those, .of theprior art, ,as 1indicatedin hxample VII. Itis believedthamhishiehs flc rat may be..due. to .thepresence. o .p resbr. mb r e e .tions,,in..the silica structure whichjcaiinot'be penetrated by nitrogengas; but which are still accessibleto' the hydroxyl... ons. 'HOW6V8 specific depolymerizationrate-canalso be ex a high plained by a less stablesilicon oxygen surface configuration. There may be other variations in i the structure which would also contribute to the high specific rate Ratio of. specific surface area as determined by nitrogen adsorptionto external specific surface area.The porosity is such that the ratio is at least 2, and ranges as high as 3, although it is probable that byvariation in batch conditions, a ratio somewhat higher than this mightbe achieved.

The following examples whereinall proportions are by weight un-less otherwise stated, illustrate specific embodiments 4 of the "invention wherein theproducts obtained are particularly characterized.

"Example 'VII Twenty grams of freshly; ground silicon as describedin' Example I, were treated withbO mini- 11 liters of 48% hydrofluorieacid in a wax-coated beaker 'byintermittent swirling for one hour. The product was filtered through filter paper in a wax-coated funnel and washed with water until the pHof the wash water rose to a constant value of 5.4. Water was removed from the wet silicon by acetone washing, the product then being dried in a nitrogen stream at 70 C. to 110 C. To two grams of this silicon, maintained in a nitrogen atmosphere, were added 75 milliliters of water and 25 milliliters of 28% aqueous ammonia, giving a solution containing 7% anhydrous ammonia. permit escape of evolved hydrogen and kept at room temperature for 15 days with intermittent shaking, at the end of which time the sol containing.2.7% silica was decanted from unreacted silicon. Forty milliliters of this sol were deionlzed with .Nalcite I-ICR resin to a pH of 4.9.

The sol was then put over Drierite" in a vac- Percent Monomer 50 In contrast to this very active sol prepared according to this invention, the following shows typical data from a material of the prior art. A sol was prepared by treating sodium silicate according to the procedure given in the co-pending application of Bechtold and'Snyder, Serial No. 65,536, filed December 15, 1948, with a. cation exchange resin togive a sol of 2% silica. This sol was then alkalized to give anSiOz/Naz=90. A heel of this sol was evaporated, maintaining a. constant volume by further addition of the same sol until at least parts of silica were added for each part of silica in the heel S01 and the concentration was built up to 30% silica.

The following data was obtained on this: 'd,=19.0 millimicrons, .dm.=16.0 millimicrons,

sc= l43 M /g, and the surface area by nitrogen adsorption is 181 M /g.

The specific depolymerization rate is as follows: 7

Percent Monomer lo Example VIII.

2 The same treated silicon was used as Ex ample VII. It was slurried, however, in- 100 milliliters of aqueous ammonia containing 3% an- This mixture was stoppered loosely to dried at 120 C. for one hour.

Percent Monomer 20 V 30 60 Two grams of freshly ground, untreated silicon as described in Example I, were treated at 20 C.-25 C. with milliliters of an aqueous solution of ammonia containing 7% anhydrous ammonia, the loosely stoppered slurry being intermittently shaken for 15 days before decantation of the sol (2.4% silica). Ammonia was removed from 30 milliliters of this sol by bubbling with nitrogen until the pH dropped from 11.15 to approximately 7. The sol was gelled by addition of acetone and the gel Was then washed free of water with acetone. The gel was then dried in air at 50 C. The specific surface area by nitrogen adsorption of this sample was 288 M /g. The specific depolymerization rate was as follows:

Percent Monomer l. 2O 30 50 60 Example X A sample of the silica sol of run 3 in Example IV, containing 3.49% silica, was, treated, after aging two years, as follows: The. pH of. the sol was reduced to 4 by addition of sulfuric acid,

causing precipitation of the silica to occur. The silica was filtered and Washed with hot concentrated hydrochloric acid and then water-Washed to a pH of approximately 2. Water was then removed by acetone washing and the product The specific surface area by nitrogen adsorption was found to be 315 M /g. The following data were obtained from electron micrographs of the sol: (18 24.0 millimocrons, dn=20.0 millimicrons, S=115 M /g.; Se (on the dried sample) :106 lVP/g. The specific depolymerization rate on the aged sol was found as follows:

ON THE SOL Percent Monomer 10 20 30 40 ON THE SAMPLE DRIED FROM ACETONE Percent Monomer l 10 20 30 4.0

The particles of silica of this invention are also characterized by having a specific surface area as determined by nitrogen adsorption which is at least twice the specific surface area as determined by the adsorption of methyl red dye. These particles of silica are also strikingly characterized by having specific depolymerization rates greater than at the point where 30% of the silica has been depolymerized to monomer.

It Will be apparent that the aqueous colloidal solution of silica of the present invention comprises silica particles markedly more porous and having much higher specific depolymerization.

rates than the silica particles of the heretofore known colloidal solutions of silica prepared by the reaction of acid with sodium silicate. The present colloidal solution of silica is distinctly new and so different from the prior silica sols as to be well adapted for uses either closed to the prior silica sols or for which the prior silica sols were so poorly adapted that they were not practically feasible.

As many apparently widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.

The invention claimed is:

1. A process for obtaining aqueous silica sols which comprises contacting finely-divided silicon metal with water which contains from about 1% to 35% by weight of ammonia and which is present in amount providing at least 0.1 gram-mol of ammonia per gram atom of silicon, and continumg the reaction at a temperature of 0 C. to 200 C. until a colloidal solution of silicon dioxide in the aqueous reaction medium is obtained.

2. The process set forth in claim 1 in which the concentration of the ammonia in the water is from 1% to 10% and in which the ratio grammols of ammonia/gram atoms of silicon is from 0.1 to 6.

3. A process for obtaining aqueous silica sols which comprises contacting, at a temperature of from 20 C. to 90 C. finely-divided silicon metal with water which contains from about 1% to 35% by weight of ammonia and which is present in amount providing at least 0.1 gram-mol of ammonia per gram atom of silicon, and continuing the reaction until a colloidal solution of silicon dioxide in the aqueous reaction medium is obtained.

4. The process set forth in claim 3 in which the concentration of the ammonia in the water is from 1% to 10% and in which the ratio grammols of ammonia/gram atoms of silicon is from 0.1 to 6.

5. A process for obtaining aqueous silica sols which comprises washing finely-divided silicon metal with aqueous hydrofluoric acid, and then contacting the finely-divided silicon metal thus treated with water which contains from about 1% to 35% by weight of ammonia and which is present in amount providing at least 0.1 gram-mol of ammonia per gram atom of silicon, and continuing the reaction at a temperature of 0 C. to 200 C. until a colloidal solution of silicon dioxide in the aqueous reaction medium is obtained.

6. An aqueous colloidal solution of silica comprising discrete spheroidal amorphous particles having an average diameter of from 8 to 35 millimicrons, the particles containing pores having a surface area at least equal to the external surface area of the particles as calculated from the surface area average particle diameter determined from electron micrographs.

7. An aqueous colloidal solution of silica comprising discrete spheroidal amorphous particles having an average diameter from 8 to 35 millimicrons, the particles containing pores having a surface area at least equal to the external surface area of the particles as determined by the adsorption of methyl red dye.

8. An aqueous colloidal solution of silicacomprising discrete spheroidal amorphous particles having an average diameter of from 8 to 35 millimicrons, the particles being further characterized by having a specific depolymerization rate greater than 20 at the point where 30% of the silica has been depolymerized to monomer, the specific depolymerization rate being calculated from the expression:

(dm/dOumXlOO A (1 )2/3 Rate from surface area average particle diameter determined from electron micrographs.

10. A silica powder comprising amorphous particles having an average diameter from 8 to 35 millimicrons, the particles containing pores having a surface area at least equal to the external surface area of the particles as determined by the adsorption of methyl red dye.

11. A silica powder comprising amorphous particles having an average diameter of from 8 to 35 millimicrons, the particles being further characterized by having a specific depolymerization rate greater than 20 at the point where 30% of the silica has been depolymerized to monomer. the specific depolymerization rate being calculated from the expression:

(dm/di) 100 A,,(lm)

where (dm/dt) 100 is the rate of silicic acid monomer formation per hundred minutes, An is the original specific surface area of the silica, and m is the fraction of total silica converted to monomer at 30 C. in 0.01 normal sodium hydroxide solution at the time, t.

JOSEPH H. BAL'IHIS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,835,420 Neundlinger Dec. 8, 1931 1,959,748 Svendson May 22, 1934 1,959,749 Svendson May 22, 1934 OTHER REFERENCES Rochow, E. C.: Chemistry of Silicones, 1946, John Wiley 8: Sons Inc., New York, p. 3. 

1. A PROCESS FOR OBTAINING AQUEOUS SILICA SOLS WHICH COMPRISES CONTACTING FINELY-DIVIDED SILICON METAL WITH WATER WHICH CONTAINS FROM ABOUT 1% TO 35% BY WEIGHT OF AMMONIA AND WHICH IS PRESENT IN AMOUNT PROVIDING AT LEAST 0.1 GRAM-MOL OF AMMONIA PER GRAM ATOM OF SILICON, AND CONTINUING THE REACTION AT A TEMPERATURE OF 0* C. TO 200* C. UNTIL A COLLOIDAL SOLUTION OF SILICON DIOXIDE IN THE AQUEOUS REACTION MEDIUM IS OBTAINED.
 6. AN AQUEOUS COLLOIDAL SOLUTION OF SILICA COMPRISING DISCRETE SPHEROIDAL ATMORPHOUS PARTICLES HAVING AN AVERAGE DIAMETER OF FROM 8 TO 35 MILLIMICRONS, THE PARTICLES CONTAINING PORES HAVING A SURFACE AREA AT LEAST EQUAL TO THE EXTERNAL SURFACE AREA OF THE PARTICLES AS CALCULATED FROM THE SURFACE AREA AVERAGE PARTICLE DIAMETER DETERMINED FROM ELECTRON MICROGRAPHS. 